Reactivity Descriptors for C-H Bond Activation and C-O Bond Formation in Hydrocarbons on Metal Oxide Catalysts

Effects of C-H bond strengths in C2 and C3 alkanes and alkenes, and H-atom abstraction strength, lattice O-atom coordination and pore confinement in transition metal oxide catalysts on product selectivity for C₂H₆-O₂ and C₃H₈-O_2­ reactions are assessed.

C-H bonds in C₂H₄ are stronger than in C₂H₆. Consequently, facile sequential C₂H₄ conversions involve C-O bond formation at vinylic carbons instead of direct C-H activation. Experiment and density functional theory (DFT) suggest that 0.4 nm one-dimensional pores in M1 phase MoVTeNb oxides enhance C₂H₄ selectivity by van der Waals stabilization of C₂H₆ C-H activation transition states and suppression of C-O bond formations. The difference in DFT derived transition state energies between C₂H₄ epoxidation (representative C-O formation reaction) and C₂H₆ C-H activation for M=O terminal O-atoms in V₂O₅ and MoVTeNbO are higher for O-atoms with less negative H-atom addition energy (HAE). This difference for M-O-M and M₂-O-M bridging O-atoms is much higher than M=O O-atoms of comparable HAE on both types of oxides. Such higher C-O bond formation activation energies are consistent with greater steric hindrances determined by greater oxide framework distortion energies for more coordinated lattice O-atoms. These analyses suggest that weaker abstractors and bridging O-atoms enhance selectivity for C₂H₆ ODH, and the inaccessibility on terminal O-atom in M1 phase pores is essential for selectivity.

C₃H₆ contains, in addition to vinylic, allylic C-H bonds that are much weaker than C-H bonds in C₃H₈. Thus, suppressing C-O formation at vinylic carbons is insufficient for hindering facile sequential C₃H₆ oxidations. Here, catalyst abstractor strength and O-atom coordination exhibit analogous effects on the preference for vinylic C-O formation over allylic C-H activation in the unavoidable secondary reactions, which leads to higher selectivity to the C-H activation mediated acrolein formation inside the pores of M1-phase.